Trim for automobiles, home housewares, toys and numerous other plastic parts are plated for appearance. In other cases the conductive layer is plated for electronic uses such as shields for electronic devices. The actual paths between the chips in some circuit boards are plated as the main conductors thus replacing wires with narrow strips or bands of copper or other metals.
Plating itself can use an electrode and requires a conductive surface that the electrolyte is deposited upon with the electrical motivating force applied externally or can be electroless where the metal is deposited from a chemically active solution. In some applications of plating on plastics the electroless plating process is used to form a thin conductive layer, then the conventional electrode plating is used to deposit a thicker layer or a different metal on the now conductive lightly plated surface of the plastic. In other cases a brief immersion in a highly unstable solution is used to form a very thin conductive layer then a thicker layer of high quality is added in a less unstable electroless plating solution.
The initial plating on plastic is usually electroless plating and can be initiated by several mechanisms. The most common method uses a catalytic coating of solutions of noble metal chlorides such as PdCl.sub.2. This common method requires that each part be processed through a dip, and the materials are very costly. In addition, the adhesion to the plated parts is limited because of the lack of bonding between the metal layer and the plastic. A common way to increase the adhesion is to mechanically roughen the surface to cause more pits and grooves. The pits and grooves are also beneficial in holding more of the catalytic agent. These surface irregularities act to trap the plated layer and thus form a mechanical bond. This is an extra step and the added roughness increases the use of catalyst solution.
Another problem with the catalyst/rough surface method occurs where a partial or localized plating is needed. The catalyst must be removed from all but the areas where the plating is desired, then masks or other shields are used to selectively deposit the plating on the exposed areas. If the unstable catalysts are not completely removed they can form unwanted electrical paths and create quality problems.
In a method to cure the poor adhesion, a vapor blast treatment of the plastic part is used. In the blasting, metallic particles are driven into the plastic as either small particles of metal or as metallic catalytic salts to form a mechanical penetration of the surface and give better adhesion in the subsequent plating step. The metallic particles are then the initiation points of the plating. This method is very labor intensive and the adhesion levels are again limited to the levels of the particle to plastic adhesion. The metals and metal chlorides used in this process can be catalytic, but this still does not increase the adhesion above the adhesion of the metals driven into the surface of the plastic. One advantage of this system is when compounds that are expensive, such as PdCl.sub.2, are used only the surface layer is treated. The problem of localized plating is still not solved with this method, and the only way to limit the plating area is with unweildly masks that are then removed. Three dimensional objects such as holes and bosses are also nearly impossible with this method because of the difficulty of impinging on such partly shaded surfaces.
A logical method to cure the lack of three dimensional plating would be the bulk dispersion of the catalyst or of a metal powder. The cost of use of the catalyst throughout the plastic has been prohibitive and the use of metal particles as a bulk filler has caused conductivity of the plastic and very low dielectric breakdown that makes this method useless for electrical applications.
In addition to the conductivity problems, the metal powders are not catalytic but react due to the differences in reactivity of the metals. This means that copper in a plating solution is exchanged for the more active iron ions in a direct substitution reaction. As seen in the recovery of copper sulphate wastes, this type reaction results in the contamination of the baths with the substituted metal. This contamination limits the life of the solutions and creates disposal problems. Since in these substitution reactions for every ion deposited an ion of the more active metal enters the plating solution, these reactions are easily indentified by the contamination in the plating solutions.
Yet another method uses an inert filler coated with a catalytic material such as PdCl.sub.2 in a thermosetting chemically cured resin. While the advantages of firmly locking the catalyst coated particles in the resin are obvious, the amount of the expensive Pd compounds used limits the use of this method. The limitation to only thermosetting resins is more serious since these cannot be processed with the ease and the speed of thermoplastic resins. Use of this technology at the temperatures involved in processing thermoplastics can destroy the unstable catalytic materials. With this method, it is nearly impossible to plate in limited areas and to plate details. The technique is, however, good for holes and for three dimensional surfaces.
A new General Electric process takes advantage of the substitution process in thermosetting matrices by adding metal powders to an ink-like plastic matrix. This method has the contamination problems mentioned above since the reaction is substitution and is limited to the thermoset or paint-like matrices. This process can be used to print patterns and reach into three dimensional objects, which makes it useful for some specialty areas and it allows the paint/polymer materials to bind to the surface of the substrate.
Most of the existing methods of providing platable plastics fail in the ability to plate fine details, and they also have either problems of adhesion to the plastic or serious cost problems. Adhesion is a major problem in uses such as automotive trim parts, door knobs and related decorative objects and the detail problems prevent combinations of graphics with plating.
The times of plating are also a concern. The substitution reactions are relatively fast with a 5 to 10 minute initial flashing followed in some cases by a much longer buildup of plating thickness after the surface is coated so no further substitution can occur. With catalytic action the rates tend to be much slower, and it make take an hour to form the initial layer and then the rate becomes about the same as the rate for the substitution reaction. It is not unusual to use a series of plating baths to optimize the rate and quality of plating.